1. Technical Field
This invention relates generally to semiconductor devices which include a plurality of stacked semiconductor die, and more particularly, to a method of constructing such a device with improved yield.
2. Background Art
Shown in FIG. 1 is a semiconductor device 20. This semiconductor device 20 is of a ball grid array (BGA) configuration which will now be described. As shown in FIG. 1, a carrier substrate 22 has a chip attach surface 24. A silicon chip or die 26 is attached to the surface 24 of the substrate 22 by a die bond 28. A plurality of solder balls 30 are attached to the substrate 22 on the side thereof opposite the die 26. The semiconductor die 26 is electrically connected to the plurality of solder balls 30 by wires 32 connecting the die 26 to traces and vias 34 through the substrate 32, which vias 34 connect to the solder balls 30. A molded package body 36 is formed over the resulting structure as shown, enclosing the die 26 and wires 32
As is well known, with the die 26 as part of a wafer, the die 26 (and the other die of the wafer) is tested by means of probes for some functions of the die 26. While such a wafer probe test with the die 26 as part of a wafer is not a complete test of all the functions of the die, it is helpful in eliminating die which prove defective in such a probe a test.
The wafer is then sawed into individual die, as is well known, and each die which has successfully been probe tested is then packaged in the manner shown in FIG. 1. The completed semiconductor device 20 is placed into a test socket 36 of a test device 38 and is clamped therein, so that the solders balls 30 are brought into contact with test probes 40 of the test socket 36. Complete functional testing is then undertaken on the die 26 of the device 20, and failed devices are eliminated.
FIG. 2 illustrates a multi-die semiconductor device 40, with the die in stacked relation. As shown in FIG. 2, a carrier substrate 42 has a chip attach surface 44. A silicon chip or die 46 is attached to the surface 44 of the substrate 42 by a die bond 48. A silicon die 50 is attached to the die 46 in stacked relation by a die bond 52. A silicon die 54 is attached to the die 50 in stacked relation by a die bond 56. A plurality of solder balls 58 are attached to the substrate 42 on the side thereof opposite the die 46. The semiconductor die 46, 50, 54 are electrically connected to the plurality of solder balls 58 by wires 60 connecting the die 46, 50, 54 to traces and vias 62 through the substrate 42, which vias 62 connect to the solder balls 58. A molded package body 64 is formed over the resulting structure as shown, enclosing the die 46, 50, 54 and wires 60.
With each die as part of a wafer, the die (and the other die of the wafer) is tested by means of probes for some functions of the die, as described above. Again, while such a probe test with the die as part of a wafer is not a complete test of all the functions of the die, it is helpful in eliminating die which have proven defective in such a wafer probe test.
Assuming all three die 46, 50, 54 to be packaged together as shown in FIG. 2 pass the wafer probe test, they are packaged as described above to provide the device 40.
The completed semiconductor device 40 is placed in a test socket 66 of a test device 68 and clamped therein, so that the solders balls 58 are brought into contact with test probes 70 of the test socket 66. Complete functional testing is then undertaken on the device 40, and failed devices are eliminated.
As noted above, a number of die which pass the wafer probe test may actually be faulty and would fail a more thorough test undertaken in a test socket. However, the die 46, 50, 54 of the device 40 are not so individually tested but rather, the overall functioning of the device 40 including die 46, 50, 54 is tested, and the device 40 is eliminated if it fails such test. The failure may be due to the failure of one of the die 46, 50, 54, with the other two die being properly functional, however the overall failure of the device 40 means that the device 40 is discarded even though two of the die may be properly functional. It should also be realized that the problem is increased with the number of the die in a package, since, due to the increased number of die in the package, there is an increased chance of including in the device a die that, while passing the wafer probe test, would actually fail more complete testing, causing the entire device to be eliminated.
Therefore, what is needed is a way to improve the yield of devices which include multiple semiconductor die.